S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 33

Introduction

Recently, biomass-derived energy has been re-ceiving more and more attention due to the deple-tion of conventional fossil sources at a faster rate, as well as major environmental issues.1 Biodiesel obtained by transesterification of vegetable oils and animal fats provides an alternative fuel option for the future.2 Transesterification of oils and animal fats produce glycerol in large excess (10 kg glycer-ol/90 kg of biodiesel) as a byproduct.3 In recent de-cades, with the rapid development of the biodiesel industry, glycerol has been oversupplied in the mar-ket, resulting in a significant decrease in the price of glycerol. Therefore, it is essential to develop a new glycerol value-added process to make the over-all biodiesel process cost-competitive and environ-mentally friendly.4,5

Various glycerol value-added processes such as esterification, hydrogenolysis, etherification, car-boxylation, fermentation, production of hydrogen and syn-gas have been reported in the literature.5–7 Among all the processes explored, esterification of glycerol with acetic acid can be a good choice of glycerol utilization. The primary products of glycer-ol esterification i.e., MAG, DAG and TAG has great industrial significance. MAG is used as an additive

in the food industry and in the manufacture of ex-plosives. DAG and TAG are used to manufacture inks, softening agents, and plasticizers.3,8,9 DAG and TAG are also useful fuel additives for reducing vis-cosity and improving anti-knocking property of gasoline.10,11 Moreover, MAG, DAG and TAG are also used in cryogenics as well as in the biodegrad-able polymer industry.12–14

Traditionally, mineral acids such as H2SO4, HCl, or H3PO4 are used as homogeneous catalysts for the glycerol esterification reaction.15–17 Howev-er, such processes are accompanied by drawbacks such as catalyst separation, product purity, necessity of neutralization, and reactor corrosion.18 There are several studies reported in the literature regarding the use of different heterogeneous catalysts for the esterification of glycerol with acetic acid.3,9,11–13,19–22 Zeolites have been reported to give poor selectivity to DAG and TAG, mostly due to their small pore size.3 Several studies have reported that the acidity of the catalyst is an important factor affecting con-version and product selectivity.9,11 Some studies have discussed the use of heteropolyacids immobi-lized on different supports like silica8, zirconia11, activated carbon12, and zeolites19. However, low se-lectivity to TAG has been reported, and some loss in activity on reuse due to leaching has been ob-served. SnCl2 was studied as a less corrosive alter-native; however, the selectivity to TAG was poor.20 A two-step reaction using acetic anhydride in the

Esterification of Glycerol with Acetic Acid over Highly Active and Stable Alumina-based Catalysts: A Reaction Kinetics Study

S. A. Rane, S. M. Pudi, and P. Biswas*Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee- 247667, Uttarakhand, India

The catalytic activity of Cu- or Ni monometallic and Cu-Ni bimetallic (Cu/Ni ra-tio = 3, 1, 0.33) catalysts supported on γ-Al2O3 and SO4

2–/γ-Al2O3 catalysts were evaluat-ed for esterification of glycerol. The reactions were performed in a batch reactor under reflux at standard reaction conditions: temperature 110 °C, atmospheric pressure, glycer-ol to acetic acid molar ratio 1:9, and catalyst loading 0.25 g. The best catalytic activity was observed over 2 M SO4

2–/γ-Al2O3 catalyst, which showed the glycerol conversion of 97 % within 2 hours of reaction. At this condition, the selectivity to glyceryl mono-acetate (MAG), glyceryl diacetate (DAG), and glyceryl triacetate (TAG) were 27.0 %, 49.9 % and 23.1 %, respectively, after 5 h of reaction. After three consecutive runs, the 2 M SO4

2–/γ-Al2O3 catalysts showed superior performance and no loss in activity was observed. The reaction kinetics results over 2 M SO4

2–/γ-Al2O3 catalyst showed that the dependence on the reaction rate to glycerol concentration was of pseudo-second order, while the activation energy was found to be 106 kJ mol–1.

Key words:glycerol esterification, Cu-Ni bimetallic catalysts, SO4

2–/γ-Al2O3 catalyst, stability, DAG and TAG selectivity

*Corresponding author: e-mail: prakbfch@iitr.ernet.in; prakashbiswas@gmail.com; tel.: (+91)-1332-28-5820; fax: (+1)-1332-27-6535

doi: 10.15255/CABEQ.2014.2093

Original scientific paper Received: August 16, 2014

Accepted: March 3, 2016

34 S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016)

second step was reported to give 100 % selectivity to TAG, but the high cost of acetic anhydride is a hindrance.13 Catalysts over SBA-3 and SBA-15 have also shown good activity for the esterification reaction.21,23 Since functionalization with the sul-phate group results in high acidity, and acidic cata-lysts are active for the esterification reaction. Sul-phation of different supports like multi-walled carbon nanotubes, zirconia, and activated carbon has been reported.9,24,25 Recently, Amberlyst and ion-exchange resins have shown very satisfactory performance on the glycerol acetylation carried out in excess of glycerol.3,10,13,26

However, the thermal stability of the catalyst, loss in active sites due to high polarity of the medi-um and low selectivity to DAG and TAG remain the main challenges in the design of a suitable catalyst. Further development of catalysts capable of provid-ing higher DAG and TAG selectivity is necessary to ensure the successful development of the industrial process for esterification of glycerol. It is very im-portant to design a highly stable, active, and selec-tive catalyst to perform the reaction in mild condi-tions.

The overall objective of this work is to develop a novel and stable catalyst system for higher DAG and TAG selectivity. In this work, we synthesized sulphated alumina with different concentrations of H2SO4, and evaluated its effect on the physicochem-ical properties of the catalyst, glycerol conversion, and product selectivity. Also examined was the per-formance of Cu or Ni monometallic and bi-metallic catalysts supported on γ-Al2O3 with different Cu/Ni weight ratios. The stability and reusability of the catalyst was investigated, and the results obtained are discussed in support of characterization results of fresh and used catalysts. This work shows that sulphated alumina is an active and stable catalyst for the acetylation of glycerol, resulting in high glycerol conversion and high selectivity to DAG and TAG. Finally, kinetics parameters were estimat-ed for the most active sulphated alumina catalyst.

Experimental details

Materials

Cu(NO3)2· 3H2O (>99 %, Himedia Chemicals, India), Ni(NO3)2· 6H2O (>99 %, Merck Specialities, India) were used as metal precursors, and γ-Al2O3 (Merck Specialities, India) was used as catalyst support. Sulphuric acid (98 %, Thomas Baker, In-dia), glycerol (99.9 %, Merck Specialities, India) and acetic acid (99.5 %, Rankem, India) were used as reactants. MAG (50 %, Alfa Aesar), DAG (50 %, Alfa Aesar), TAG (99 %, Alfa Aesar), and n-butanol

(99 %, Rankem, India) were procured and used to prepare the calibration plot.

Catalyst synthesis

Synthesis of Cu-Ni catalysts

20 wt % Cu/γ-Al2O3, 20 wt% Ni/γ-Al2O3, and 20 wt% Cu-Ni/γ-Al2O3 catalysts were prepared by the wetness impregnation method.22 Pre-calculated amount of precursors Cu(NO3)2· 3H2O and Ni (NO3)2· 6H2O were dissolved in double-distilled water. To this metal precursor solution, γ-Al2O3 sup-port was added slowly while stirring. Three differ-ent weight ratios (Cu/Ni = 3, 1, and 0.33) were se-lected for the synthesis of bimetallic catalysts. The slurry was aged for 24 h at room temperature and then oven-dried for 12 h at 110 °C, followed by cal-cination in air at 400 °C for 4 h. The catalysts were labelled as Cu/γ-Al2O3, Ni/γ-Al2O3, Cu-Ni (1:3)/γ-Al2O3, Cu-Ni (1:1)/γ-Al2O3, and Cu-Ni (3:1)/γ-Al2O3, respectively.

Synthesis of sulphated alumina

Sulphated alumina catalysts were prepared by following a method reported in literature.25 Alumina was treated with sulphuric acid solution at three diffe-rent concentrations (0.2 M, 2 M and 4.8 M), and the treated alumina was aged for 5 h at room temperatu-re, followed by drying at 110 °C for 24 h, and cal-cination at 550 °C for 3 h in air. The sulphated alu-mina catalysts were labelled as 0.2 M SO4

2–/γ-Al2O3, 2 M SO4

2–/γ-Al2O3 and 4.8 M SO42–/γ-Al2O3, respec-

tively.

Catalyst characterization

The N2 adsorption-desorption isotherm and surface area of the catalysts were determined by nitrogen adsorption at liquid nitrogen temperature using the Micromeritics ASAP-2020 instrument. Prior to the measurement, the samples were de-gassed in a vacuum at 200 °C for 4 h. Pore size distribution was calculated by the BJH (Bar-ret-Joyner-Halenda) method using the desorption branch of isotherm.

The XRD spectra of catalysts were obtained using a Bruker AXS D8, diffractometer with a ­Cu-Kα­radiation­(λ = 0.154 nm) source. All the data were recorded at the two theta interval of 10–80° with a steep of 0.02° s–1 at 3 s time constant. The Scherrer equation was used to calculate the average crystallite size of the metal particles using the XRD line broadening technique.

Temperature programmed desorption (TPD) with ammonia was carried out to determine the acidic property of the catalyst. The NH3-TPD ex-periments were carried out using Micromeritics

S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 35

Chemsorb 2720 apparatus equipped with a thermal conductivity detector. Prior to the TPD experiments, the catalyst samples were pre-treated at 150 °C for 2 h in a flow of helium. After pre-treatment, the cat-alyst samples were allowed to cool at room tem-perature. Then the samples were saturated with 27 % NH3/He gas mixture for 1 h at a flow rate of 30 mL min–1. To remove the excess ammonia, the samples were flushed with helium. The ammo-nia-TPD was performed using 20 cc min–1 of helium, heating the samples from 30 °C to 700 °C at a heating rate of 10 °C min–1 while continuous-ly monitoring the thermal conductive detector sig-nals.

FT-IR analysis of sulphated alumina catalysts were recorded on a Thermo Nicolet (Model magna 760) spectrometer at room temperature in KBr pel-lets over a range of 500 to 4000 cm–1 under atmo-spheric conditions.

The leaching of metal from the support into the liquid medium during recycle experiments was test-ed by using inductively coupled plasma mass spec-trometry (ICP-MS).

Catalytic evaluation

The esterification reactions were carried out at 110 °C and atmospheric pressure in a round bottom flask equipped with a reflux and a magnetic stirrer. Glycerol to acetic acid molar ratio of 1:9 (total vol-ume of 30 mL) and catalyst loading of 0.25 g was maintained for all reactions. In all the experi-ments, stirrer speed was kept constant at 700 rpm. Samples cooled to room temperature, were taken periodically, filtered, and analysed by gas chroma-tography.

Product analysis

A Newchrom GC (Model: 6800, India) equipped with AB-PONA (stationary phase, 50 m × 0.2 mm) column equipped with a flame ionization detector (FID) was used to detect product components such as MAG, DAG and TAG. Oven temperature was kept constant at 200 °C. The FID and injector tem-peratures were fixed at 280 °C and 260 °C, respec-tively. For calculating product selectivity, n-butanol was used as the internal standard. The glycerol con-version and product selectivity were defined as pro-posed by Zhou et al.27

Conversion of glycerol or acetic acid (%) = [1 – (moles of glycerol or acetic acid remaining) / initial moles of glycerol or acetic acid)]· 100

Selectivity of MAG, DAG and TAG (%) = [(moles of MAG, DAG or TAG formed) / (total moles of MAG + DAG + TAG)]· 100

Results and discussion

Catalyst characterization

The surface area, total pore volume, and the average pore diameter of catalysts are given in Ta-ble 1. The surface area of γ-Al2O3 was 107 m2 g–1, and decreased ~35 % after metal impregnation and was in the range of 67–77 m2 g–1. The reduction in surface area of the catalysts was due to the structur-al collapse of precursor during calcinations.28 The surface area of SO4

2–/γ-Al2O3 catalysts decreased significantly (52.1–2.4 m2 g–1) when H2SO4 concen-tration increased from 0.2 M to 4.8 M. This result might be due to the blockage of pores by the active SO4

2– species.29 The pore volume of γ-Al2O3 support-ed Cu or Ni and Cu-Ni catalysts were in the range of 0.12–0.20 cm–3 g–1, whereas, for SO4

2–/γ-Al2O3 catalysts it reduced significantly (0.002–0.092 cm3 g–1). The pore size of all catalysts were calculated by ap-plying the BJH model for desorption isotherm data. The result shows that the average pore diameter of the Cu-Ni catalysts is lower (55–59 Å) than that of the SO4

2–/γ-Al2O3 catalysts (63–118 Å). Moreover, the average pore diameter passed through minima with increasing SO4

2– concentration on γ-Al2O3.

Ta b l e 1 – Structural parameter, specific surface area, and acidity of the catalysts

Catalyst SBET (m2 g–1)

Vp (cm3 g–1)

Dp (Å)

Acidity (mmol NH3 g cat–1)

γ-Al2O3 107 0.20 57 1.20

Cu/γ-Al2O3 77 0.15 59 0.56

Cu-Ni(3:1)/γ-Al2O3 67 0.13 57 0.70

Cu-Ni(1:1)/γ-Al2O3 78 0.15 56 0.61

Cu-Ni(1:3)/γ-Al2O3 69 0.12 55 0.89

Ni/γ-Al2O3 71 0.12 55 0.41

0.2 M SO42–/γ-Al2O3 52.1 0.092 62.9 1.32

2 M SO42–/γ-Al2O3 8.4 0.012 38.3 2.51

4.8 M SO42–/γ-Al2O3 2.4 0.002 117.6 1.98

The XRD patterns of calcined Cu/γ-Al2O3, Ni/γ-Al2O3 and Cu-Ni/γ-Al2O3 catalysts are shown in Fig. 1A. For the Cu/γ-Al2O3 catalyst, the diffrac-tion peaks were obtained at 2θ = 35.5°, 38.7°, 48.6° and 61.5°, corresponding to (111), (111), (202) and (113) planes of monoclinic CuO (JCPDS: 41-0254). A broad peak was observed for all the catalysts at 2θ = 67.5° corresponding to Al2O3 (JCPDS: 86-1410). The intensity of the peaks detected for Cu/γ-Al2O3 catalysts indicates that CuO is of good crystalline nature. The XRD pattern of the Ni/γ-Al2O3 catalyst

36 S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016)

shows sharp reflections at 2θ = 37.3°, 43.2° and 62.8°, which corresponds to NiO (JCPDS: 44-1159), and reflects typical rhombohedral structure. For Cu-Ni/γ-Al2O3 bimetallic catalysts, additional peaks were observed at 2θ = 36.9°, 43.2° and 62.8° other than CuO. The diffraction peak at 2θ = 36.9° corresponds to the mixed metal oxide phase Cu0.5Ni0.5Al2O4 (JCPDS: 78-1603), and peaks at 2θ = 43.2° and 62.8° correspond to (012) and (110) planes of NiO (JCPDS: 44-1159). The XRD pattern of Cu-Ni bimetallic catalysts shows that the intensi-ty of the peaks corresponding to the mixed metal oxide phase (Cu0.5Ni0.5Al2O4) and NiO increased with Ni loading. The intensity of the peaks corre-sponding to CuO and NiO for bimetallic catalysts were low and broad compared to that for monome-tallic catalysts. This result indicates better disper-sion of metal on alumina support for bimetallic cat-alysts.

XRD patterns of sulphated alumina catalysts are shown in Fig. 1B. For 0.2M SO4

2–/γ-Al2O3 cata-lyst, the peaks obtained at 2θ = 44.6° and 67.4° cor-respond to alumina, which confirms the presence of the monoclinic phase of Al2O3 (JCPDS: 86-1410).

For this catalyst, no additional peaks were detected for SO4

2– species. However, for 2 M SO42–/γ-Al2O3

and 4.8 M SO42–/γ-Al2O3 catalysts, an additional

peak was obtained at 2θ = 25.5° corresponding to Al2(SO4)3 (JCPDS: 81-1835).29 The intensity of the peak corresponding to Al2(SO4)3 increased with the concentration of SO4

2– in the catalyst.A quantitative estimation of the total acidity

of the catalysts based on desorbed ammonia at dif-ferent temperature is summarized in Table 1. The NH3-TPD profiles (Fig. 2A) of the Cu/γ-Al2O3, Ni/γ-Al2O3 and Cu-Ni/γ-Al2O3 catalysts shows NH3 desorption peaks in three different temperature regions for all catalysts. The peaks obtained at 80–250 °C are attributed to desorption of ammonia from weak acidic sites, and the peaks at 250–500 °C refer to moderate strength acidic sites on the cata-lysts. The high temperature (> 500 °C) peaks indi-cate that strong acid sites exist on the catalysts.30,31 The NH3-TPD profile of the γ-Al2O3 surface displays the peaks in all three temperature ranges, and suggests the high acidic strength of 1.203 mmol NH3 g cat.–1. The results reported in Table 1 suggest that the acidic strength of the catalysts had decreased signi fi cantly after metal impregnation. Monometallic and bime-

F i g . 1 – XRD patterns of (A) Cu/γ-Al2O3, Ni/γ-Al2O3 and Cu-Ni/γ-Al2O3 catalysts (B) SO4

2–/γAl2O3 catalysts

F i g . 2 – NH3-TPDprofilesof(A) γ-Al2O3,Cu/γ-Al2O3, Ni/γ-Al2O3 andCu-Ni/γ-Al2O3 catalysts (B) SO4

2–/γ-Al2O3 catalysts

S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 37

tallic catalyst samples have acidity in the range of 0.407-0.562 mmol NH3 g cat–1

, which is lower than the acidity (1.203 mmol NH3 g cat–1) of γ-Al2O3.

The NH3-TPD results obtained for the SO4

2–/γ-Al2O3 catalysts and γ-Al2O3 are shown in Fig. 2B. The TPD profiles obtained for all the cata-lysts indicate widely distributed surface acidic strength.11 For 0.2 M SO4

2–/γ- Al2O3 catalyst, the de-sorption peak was detected at high temperature (500–750 °C), which suggests the existence of strong acid sites on the catalysts. For 2 M SO4

2–/γ-Al2O3 and 4.8 M SO4

2–/γ-Al2O3 catalysts, a major broad peak with high intensity was detected at 250–500 °C, indicating medium acidic sites.27 The intensity of the peak obtained for both these catalysts is almost identical. Similar results have also been reported by Yang et al.29 They found that the intensity of strong acidic peaks remained unchanged for [H2SO4]­≥­2.4­M.­The estimated total acidity of the catalysts showed that the impregnations of H2SO4 on γ-Al2O3 resulted in an increase in acidity (Table 1). The acidity of 0.2 M SO4

2–/γ-Al2O3 is higher (1.32 mmol NH3 g cat–1) than that of γ-Al2O3 (1.203 mmol NH3, g cat–1). The highest acidity of 2.51 mmol NH3 g cat–1 was ob-tained for 2 M SO4

2–/γ-Al2O3 catalyst.The FT-IR spectra (Fig. 3) of sulphated alumi-

na catalysts displayed the bands in the wave num-ber range of 3000–3400 cm–1 and at ~1632 cm–1 corresponding to the hydroxyl groups.32 However, the bands at ~1100 cm–1 correspond to the symmet-ric vibrations of S = O group.33 Increased sulphate content enhances the intensity of the bands and shifts the characteristic vibration of S = O groups towards higher values (~1200 cm–1). This shift is mostly due to the formation of new sulphate species in case of such sulphate loading.34

Catalytic evaluation

To verify the homogeneous reaction, esterifica-tion reactions were performed in the absence of cat-alysts (blank test) by keeping the other reaction conditions constant, and the results show that, in the absence of catalyst, glycerol conversion is signifi-cant (82 %) (Table 2). In the blank test, MAG (71.5 %) and DAG (26.5 %) were obtained as the two main reaction products. However, a trace amount of TAG (2 %) was also detected. Depending on the catalyst used, this reaction is accelerated in different ways; as a result, the glycerol conversions and product selectivity obtained are quite different for different catalysts. However, MAG, DAG and TAG are the main reaction products, these products

F i g . 3 – FT-IRpatternsofSO42–/γ-Al2O3 catalysts (A) 0.2 M

SO42–/γ-Al2O3 (B) 2 M SO4

2–/γ-Al2O3 (C) 4.8 M SO4

2–/γ-Al2O3

Ta b l e 2 – Catalytic behavior for esterification of glycerol with acetic acida

Catalyst Glycerol conversion (%)

Selectivity (%) Yield TAG (%) Initial reaction rateb

(mmolgly g cat–1 h–1)MAG DAG TAG

Blank 82 71.5 26.5 2.0 1.7 –

γ-Al2O3 82 86.7 12.9 0.5 0.4 0.90

Cu/γ-Al2O3 84 70.7 27.0 2.3 1.9 0.99

Cu-Ni(3:1)/γ-Al2O3 89 75.4 23.2 1.4 1.2 3.11

Cu-Ni(1:1)/γ-Al2O3 94 77.6 21.2 1.2 1.1 2.16

Cu-Ni(1:3)/γ-Al2O3 97 68.9 28.5 2.5 2.5 2.32

Ni/γ-Al2O3 97 68.9 28.4 2.6 2.6 1.79

0.2 M SO42–/γ-Al2O3 97 30.5 53.6 15.9 15.5 3.61

2 M SO42–/γ-Al2O3 97 27.0 49.9 23.1 22.5 4.22

4.8 M SO42–/γ-Al2O3 97 33.0 48.8 18.2 17.7 3.90

aReaction conditions: glycerol/acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reaction time = 5 h, catalyst weight = 0.25 g.bInitial reaction rate is calculated for all catalysts after 0.5 h of reaction.

38 S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016)

formed according to the reaction scheme reported earlier.3,7 In the presence of catalysts, the reaction path of esterification of glycerol involves the con-version of glycerol to MAG, whereas DAG and TAG form through consecutive acetylation reac-tion.23 The glycerol conversion with the sulphated alumina catalyst is highest (97 %) and the effect of the glycerol conversion with the concentration of sulphate species is almost negligible. Among all the γ-Al2O3 supported metal oxide catalysts, Ni/γ-Al2O3 and Cu-Ni (1:3)/γ-Al2O3 catalyst shows the highest glycerol conversion of 97 %. Similar results have been reported earlier for hydrogenolysis of glycer-ol.35 This result demonstrates that the impregnation of metal has a positive effect on the glycerol esteri-fication reaction. However, for the Cu/γ-Al2O3 cata-lyst, glycerol conversion is low (85 %). In bimetal-lic catalysts, after increasing the Ni content in the catalyst, the glycerol conversion increased from 89 % to 97 %. Kang et al.36 reported that, with in-creasing nickel content in the Cu-Ni/γ-Al2O3 cata-lysts, the catalytic activity had increased for hy-drogenolysis of 1,3-butadiene. By comparing the structural parameter and acidity of the catalysts ( Table 1), it can be observed that the catalytic activ-ity is independent of the structural properties and the acidity of supported metal oxide catalysts.9 In the presence of γ-Al2O3, glycerol conversion is 82 %, which is similar to the conversion obtained for uncatalyzed reaction. This result indicates that γ-Al2O3 itself has no significant catalytic activity for esterification of glycerol with acetic acid.

Variation of glycerol conversion with time for SO4

2–/γ-Al2O3 catalysts is plotted and shown in Fig. 4. The results show that the initial activity of the cata-lysts passed through maxima with H2SO4 concentra-tion. After 30 minutes, for 0.2 M SO4

2–/γ-Al2O3 cat-

alysts, glycerol conversion was ~74 %, then activity increased to ~87 % for 2 M SO4

2–/γ-Al2O3 catalysts, and further decreased to ~80 % in case of 4.8 M SO4

2–/γ-Al2O3 catalysts. The initial increase in activ-ity with the increase in concentration of H2SO4 can be explained by kinetic effect. While at higher con-centrations, the decrease in activity might be due to the limitations of internal diffusion in the catalyst.3 Glycerol conversion increased rapidly in the first hour, after which the rate of increase slowed down. For all three SO4

2–/γ-Al2O3 catalysts, glycerol con-version exceeded 85 % within 1 h of the reaction, and reached 97 % after 5 h of reaction. The 2 M SO4

2–/γ-Al2O3 catalysts show the best catalytic activ-ity and 97 % glycerol conversion is achieved after 2 h of reaction. The results obtained for sulphated alumina catalysts can be correlated with the total acidity of the catalysts. The highest activity of 2 M SO4

2–/γ-Al2O3 catalysts is due to the high acidic strength of 2.51 mmol NH3 g cat.–1 (Table 1).

The initial reaction rate was calculated for all the catalysts after 0.5 h of the reaction and the values obtained are shown in Table 2. The calculated reaction rate is in the range of 0.9–4.22 mmolgly g cat–1 h–1. The highest initial reaction rate of 4.22 mmolgly g cat–1 h–1 was obtained for 2 M SO4

2–/γ-Al2O3 catalyst.Fig. 5 compares the glycerol conversion with

time for Ni/γ-Al2O3, 2 M SO42–/γ-Al2O3 catalyst and

uncatalyzed reaction. It can be observed that, in the presence of catalyst, glycerol conversion is signifi-cantly higher than the uncatalyzed reaction. Where-as the glycerol conversion stabilized at 97 % for Ni/γ-Al2O3 and 2 M SO4

2–/γ-Al2O3 catalyst after 5 h of reaction, the rate of reaction was significantly higher with 2 M SO4

2–/γ-Al2O3 catalyst. For 2 M SO4

2–/γ-Al2O3 catalyst, 97 % glycerol conversion

F i g . 4 – Variation of glycerol conversion with time over SO4

2–/γ-Al2O3 catalysts. Reaction conditions: Glycerol/Acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reac-tion time = 5 h, catalyst weight = 0.25 g.

F i g . 5 – Comparison of glycerol conversion with time over different catalysts. Reaction conditions: Glycerol/Acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reaction time = 5 h, catalyst weight = 0.25 g.

S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 39

was achieved after 2 h of reaction, while for Ni/γ-Al2O3 catalyst, it took about 4 h to reach 90 % glycerol conversion. This result follows the calcu-lated reaction rate data (Table 2).

For all the catalysts, the main reaction products are MAG, DAG and TAG. Table 2 shows the selec-tivity to MAG, DAG and TAG obtained after 5 h of reaction. The results demonstrate that the catalyst used affects the product selectivity. The results

obtained over metal-impregnated alumina (Cu or Ni and Cu-Ni) suggest that these catalysts have a slightly positive effect on the product selectivity. Similar product distribution was observed for Ni/γ-Al2O3 and Cu:Ni(1:3)/γ-Al2O3 catalyst, and after 5 h of reaction show ~68.9 %, ~28.5 % and ~2.5 % selectivity to MAG, DAG and TAG, respec-tively. This result suggests that Ni/γ-Al2O3 and Cu:Ni(1:3)/γ-Al2O3 catalyst is more selective to MAG but not very effective for successive acetyla-tion of MAG to DAG and TAG.

The product selectivity obtained over sulphated alumina (Fig. 6) show that sulphation has a signifi-cant effect on the selectivity of products. For all the sulphated catalysts, DAG and TAG selectivity in-creased significantly with time at the expense of MAG. The 2 M SO4

2–/γ-Al2O3 catalyst showed the best results in terms of product selectivity and for this catalyst, MAG, DAG and TAG selectivity were 27.0 %, 49.9 % and 23.1 %, respectively. This re-sult suggests that successive acetylation of MAG is favorable in the presence of sulphated alumina. The results obtained over 2 M SO4

2–/γ-Al2O3 catalyst are comparable with the result reported in the litera-ture.3,8 However, in this study, the obtained selectiv-ity to TAG is significantly higher (23.1 %). Previ-ously, Liao et al.17 reported ~100 % glycerol conversion with ~25 % selectivity to TAG after 4 h of reaction over Amberlyst-35 catalyst. Com-paring our results with the results reported by Liao et al.17, it could be clearly observed that over 2 M SO4

2–/γ-Al2O3 catalyst, 97 % glycerol conversion with 23 % selectivity to TAG was obtained in the presence of lower catalyst concentration.

The product distribution obtained over sulphat-ed alumina catalysts suggests that DAG and TAG selectivity had an increasing trend, whereas MAG selectivity had a decreasing trend with reaction time. This suggests that, initially, glycerol convert-ed to MAG, and as the reaction progressed, MAG further converted to DAG and TAG through succes-sive acetylation reaction. However, the feasibility of this consecutive acetylation reaction depends ex-tremely on the type of catalysts used. The 2 M SO4

2–/γ-Al2O3 catalyst showed the highest selectivi-ty to DAG (49.9 %) and TAG (23.1 %) after 5 h of reaction. The schematic diagram of the reaction pathways is shown in Fig. 7.

F i g . 6 – Variation of MAG, DAG and TAG selectivity withtime for SO4

2–/γ-Al2O3 catalysts Reaction conditions: Glycerol/Acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reaction time = 5 h, catalyst weight = 0.25 g.

40 S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016)

To verify the effect of bare H2SO4 for esterifi-cation of glycerol, an experiment was conducted with H2SO4 instead of using 2 M SO4

2–/γ-Al2O3 as catalyst, maintaining all other reaction conditions constant. The results obtained are shown in Table 3. The results demonstrate that, in the presence of H2SO4, 84 % glycerol conversion was achieved

after 5 h of reaction, and the main acetylation prod-ucts were MAG and DAG. Moreover, the selectivi-ty to TAG was almost negligible (0.8 %), while the selectivity to MAG and DAG was 38.1 % and 68.1 %, respectively. This result agrees with an ear-lier report.37

The best results obtained over 2 M SO42–/γ-Al2O3

catalyst was also compared with the catalytic activ-ity of other sulphated catalysts reported in literature, and the comparison data are presented in Table 3. It can be observed that sulphated alumina catalysts show much higher selectivity to more desirable DAG and TAG.

Stability and reusability tests

Deactivation behavior and reusability of Ni/γ-Al2O3 and 2 M SO4

2–/γ-Al2O3 catalysts were exam-ined at 110 °C and atmospheric pressure with a feed mixture containing glycerol and acetic acid (1:9 mole ratio) for 5 h. Prior to each cycle of the exper-iment, the catalyst was separated by centrifugation, washed with deionised water, and then dried over night at 110 °C. The results obtained for three suc-cessive reactions are summarized in Table 4. For Ni/γ-Al2O3 catalyst, a loss in catalytic activity was observed in every consecutive run, and glycerol conversion reduced from 97 % to 88 % in succes-sive cycles, while the product selectivity remained almost unchanged. This decrease in conversion was due to the leaching (~10–15 %) of nickel metal into the liquid phase confirmed by the ICP-MS analysis. For 2 M SO4

2–/γ-Al2O3, no loss in catalytic activity was observed and glycerol conversion was main-tained at 97 % even after cycle-2. The selectivity of products was also consistent with the successive re-cycle experiments. In contrast to these results, the

F i g . 7 – Reaction pathway for acetylation of glycerol over 2 M SO42–

/γ-Al2O3 catalyst

Ta b l e 3 – Catalytic behavior over different catalysts

Catalyst Glycerol conversion (%)

Selectivity (%)Yield of TAG (%) Reference

MAG DAG TAG

0.2 M SO42–/γ-Al2O3

a 97 30.5 53.5 15.8 15.5 This study

2 M SO42–/γ-Al2O3

a 97 27.0 49.9 23.1 22.5 This study

4.8 M SO42–/γ-Al2O3

a 97 32.9 48.8 18.2 17.7 This study

H2SO4a 84 38.1 61.1 0.8 0.6 This study

H2SO4b 98 54 27 Traces Traces [20]

Sulphated zirconiac – 98 2 Traces Traces [21]

Sulphated activated carbond 91 38 28 34 31 [23]aReaction conditions: glycerol/acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reaction time = 5 h, catalyst wt. = 0.25 g.bReaction conditions: glycerol/acetic acid (molar ratio) = 1:3, reaction temperature = 60 °C, reaction time = 8 h, catalyst conc. (H+) = 0.03 mmol.cReaction conditions: glycerol reaction temperature = 55 °C, reaction time = 24 h, catalyst weight = 250 mg/40 mL glycerol; monoacetin, 40 %, concentration = 32.4 g L–1 glycerol.dReaction conditions: glycerol/acetic acid (molar ratio) = 1:9, reaction temperature = 120 °C, reaction time = 3 h, catalyst wt.= 0.8 g.

S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 41

previous studies over sulphated catalysts showed a significant decrease in glycerol conversion and TAG selectivity due to the lower stability of sul-phonic species in the reusability test.9,23 In this study, the results obtained in the reusability experi-ment suggest that the 2 M SO4

2–/γ-Al2O3 catalyst was very stable and showed consistent performance in consecutive runs.

Characterization of used catalysts

To verify the structural stability of the cata-lysts, samples of the used catalyst were character-ized after each experiment. XRD patterns of fresh and used Ni/γ-Al2O3 catalysts were compared. From Fig. 8, it can be seen that the intensity of the peaks corresponding to NiO at 2θ = 43° and 37° had de-creased in every successive use. This result suggests the loss of metal from the catalyst surface into the liquid due to leaching during the reaction. To con-firm the metal leaching, the reaction mixture was collected after 5 h of reaction, and the metal content analyzed by ICP- MS analysis. The results obtained

confirmed 10–15 % leaching of nickel into the reac-tion mixture.

The FT-IR spectra of the used 2 M SO42–/γ-Al2O3

catalyst after every successive cycle were compared with the FT-IR of the fresh catalyst (Fig. 9). The result shows that the peak corresponding to sulphate species at around 1100 cm–1 is intact in the used cat-alyst. The additional peaks detected in the used cat-alyst at 1735 cm–1 correspond to the esters formed in the reaction, and at 1360 cm–1 to alcohols.33 Therefore, 2 M SO4

2–/γ-Al2O3 can be considered a stable catalyst for the esterification of glycerol with acetic acid.

Kinetics of glycerol esterification

Studies of the reaction kinetics for glycerol acetylation are very limited in literature. The kinetic study of the esterification of glycerol with acetic acid catalyzed by 2 M SO4

2–/γ-Al2O3 was carried out by conducting the reaction at different temperatures, and collecting and analyzing reaction samples at regular intervals (Fig. 10 and Fig. 11). Other param-

Ta b l e 4 – Catalytic behavior of catalyst during recycling experimentsa

Catalyst Experiment Glycerol conversion (%) MAG selectivity (%) DAG selectivity (%) TAG selectivity (%)

Ni/γ-Al2O3

Fresh 97 68 29 3

Cycle 1 92 65 32 3

Cycle 2 88 69 29 2

2 M SO42–/γ-Al2O3

Fresh 97 32 47 21

Cycle 1 97 35 46 19

Cycle 2 97 35 47 18aReaction conditions: glycerol/acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reaction time = 5 h, catalyst weight = 0.25 g.

F i g . 8 – XRDpatternsoffreshandusedNi/γ-Al2O3 catalysts, (A) Fresh catalyst, (B) Cycle-1, (C) Cycle-2, (D)Cycle-3

F i g . 9 – FT-IR spectra of fresh and used 2M SO42–/γ-Al2O3

catalyst,(A)Freshcatalyst,(B)Cycle-1,(C)Cycle-2,(D)Cycle-3

42 S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016)

eters like pressure, catalyst loading, and the molar ratio of reactants that may affect kinetics were kept constant. In this study, the Arrhenius equation was employed to determine the rate constant at different temperatures, estimate order of the reaction, and de-termine the activation energy for the reaction. The glycerol conversion trend obtained over SO4

2–/γ-Al2O3 catalyst suggested that the glycerol concentration changed rapidly in the first 2 h, after which the glycerol concentration stabilized (Fig. 4). There-fore, for the purpose of kinetic study, the reaction was carried out for 2 h, and samples were collected at intervals of 30 minutes. Fig. 10 shows the varia-tion of glycerol concentration with time at different temperatures.

As the esterification reaction was performed by reacting glycerol (G) with excess amounts of acetic acid (A), it could be assumed that the concentration of acetic acid remained almost constant throughout

F i g . 1 0 – Effect of temperature on glycerol concentration for 2 M SO4

2–/γ-Al2O3 catalyst. Reaction conditions: Glycerol/Acetic acid (molar ratio) = 1:9, reaction temperature = 110 °C, reaction time = 5 h, catalyst weight = 0.25 g.

F i g . 11 – Kinetic curves obtained for esterification of glycerol over 2 M SO42–/γ-Al2O3catalyst,(A)&(B)first-order-dependence;

(C)&(D)second-order-dependence

S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 43

the reaction, and the rate could be expressed in terms of the concentration of glycerol alone. The generalized reaction rate can be expressed as

− =− =rCt

k C CGG

Gx

Ayd

d 1 (1)

Initially, a pseudo-first-order reaction was as-sumed. Since acetic acid (A) was present in excess, the concentration of glycerol (G) was the limiting reactant. Therefore, the reaction rate would have depended on the concentration of glycerol, thus, the proposed pseudo-first-order reaction rate can be written as follows:

− =− =rCt

k CGG

Gdd 1 (2)

On integration, Eq. (2) reduces to

lnCC

k tG

G01

=− (3)

The plots of ln(CG/CG0) were obtained, as pre-sented in Fig. 11A, where CG0 is the initial concen-tration of glycerol and CG is the concentration of glycerol at any given time. The slope of the straight line fit to the curves is (k), the rate constant for the reaction at the particular temperature. The values of rate constant (k) obtained as the slope of each plot and the linearity coefficients are listed in Table 5. The effect of variation of temperature on the rate constant was used to calculate activation energy by employing the Arrhenius equation.

k AeERTa

=−

(4)

Where, k = rate constant, A = Arrhenius con-stant, Ea = Activation energy, R = Universal gas constant, and T = temperature.

Ta b l e 5 – Rateconstant(k)forpseudo-first-orderandpseudo--second-orderdependenceonglycerolconversion

Temperature (°C)

Pseudo-first-order dependence

Pseudo-second-order dependence

k (s–1) regression k (L mol–1 s–1) regression

70 1.10 · 10–4 0.93 1.99· 10–7 0.95

90 3.42 · 10–4 0.99 1.50· 10–6 0.87

110 6.04 · 10–4 0.89 9.89· 10–6 0.97

Employing the equation (4), it is clear that the slope of a plot of (ln k) versus 1/T will be equal to (Ea/R). The plot of (ln k) versus 1/T is shown in Fig. 11B. The activation energy of 2 M SO4

2–/γ-Al2O3 catalyzed esterification of glycerol is found to be

47.13 kJ mol–1. From Fig. 11B, it can be seen that, for pseudo-first-order fit, the R2 value is ~0.95.

Further, the pseudo-second-order dependence to glycerol concentration was also studied for better fit. A similar procedure was followed as reported by Goncalves et al.37 to test second-order dependence. Assuming a pseudo-second-order dependence to glycerol concentration, the reaction rate can be written as follows:

− =− =rCt

k CGG

Gdd 1

2 (5)

On integration, Eq. (5) reduces to

1 1

02C Ck t

G G− = (6)

The plots of (1/CG – 1/CG0) versus time were ob-tained as presented in Fig. 11C, where CG0 is the initial concentration of glycerol, and CG is the concentration of glycerol at any given time. The slope of straight line fit to the curves is (k), the rate constant for the reaction at the particular temperature. The values of rate con-stant (k) obtained as the slope of each plot and the linearity coefficients are listed in Table 5.

The effect of temperature on rate constant is plotted in Fig. 11D, and activation energy was cal-culated by employing the Arrhenius equation, as before. The activation energy of 2 M SO4

2–/ γ-Al2O3 catalyzed esterification of glycerol, assuming pseu-do-second-order dependence to glycerol concentra-tion was found to be 106.62 kJ mol–1, which is close to the value (101 kJ mol–1) reported by Zhou et al.26 For pseudo-second-order fit, the R2 value was ~99 % (Fig. 11D). This result suggests that the reaction rate to glycerol concentration is pseudo-second-or-der over 2 M SO4

2–/γ-Al2O3 catalyst.

Conclusions

SO42–/γ-Al2O3 and Cu-Ni/γ-Al2O3 catalysts

demonstrate good activity towards acetylation of glycerol and yielded MAG, DAG and TAG as the main reaction products. Among all the supported metal catalysts examined, Cu-Ni(1:3)/γ-Al2O3 and Ni/γ-Al2O3 catalysts showed the maximum glycerol conversion of 97 % after 5 h of reaction. For these catalysts, MAG, DAG and TAG selectivity was ~68.9 %, ~28.5 % and ~2.5 %, respectively. This result suggests that these catalysts were more selec-tive to MAG, but not very effective for successive acetylation of MAG to DAG and TAG.

Sulphation of γ-Al2O3 significantly affected the catalytic activity and product selectivity. For all the SO4

2–/γ-Al2O3 catalysts, >85 % glycerol conver-sion was achieved within 1 h of reaction, and after

44 S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016)

3 h, 97 % glycerol conversion was obtained. For SO4

2–/γ-Al2O3 catalysts, DAG and TAG selectivity showed an increasing trend and selectivity to MAG showed a decreasing trend with time. Product distri-bution obtained for SO4

2–/γ-Al2O3 catalysts suggest-ed that, initially, glycerol converted to MAG, and as the reaction progressed, MAG further converted to DAG and TAG through successive acetylation reac-tion over these catalysts. Therefore, SO4

2–/γ-Al2O3 catalysts were highly selective to DAG and TAG. Among all the sulphated catalysts examined, 2 M SO4

2–/γ-Al2O3 catalyst was found to be the most active catalyst resulting in a glycerol conversion of 97 % within 2 h of reaction. Maximum ~49.9 % selectivity to DAG and ~23.1 % selectivity to TAG was obtained over 2 M SO4

2–/γ-Al2O3 catalyst after 5 h of reaction.

Stability of the catalysts was examined by con-ducting reusability tests with three consecutive ex-periments. Over 2 M SO4

2–/γ-Al2O3 catalyst, no loss in activity was observed even after three consecu-tive experiments, and the catalyst could be consid-ered stable for the esterification of glycerol with acetic acid. Finally, kinetic measurements over 2 M SO4

2–/γ-Al2O3 catalyst showed that the dependence on the reaction rate to glycerol concentration was pseudo-second-order. The activation energy for 2 M SO4

2–/γ-Al2O3 catalyst was found to be 106 kJ mol–1.

ACKNOWLEDGEMENTS

TheauthorsarethankfultotheMinistryofHu-manResourceDevelopment (MHRD),Govt. of In-dia for awarding the fellowship to carry out this work in the Department of Chemical Engineering at theIndianInstituteofTechnology,Roorkee.

R e f e r e n c e s

1. Bozell, J., Connecting biomass and petroleum processing with a chemical Bridge, Science 329 (2010) 522.doi: http://dx.doi.org/10.1126/science.1191662

2. Xia, S., Zheng, L., Wang, L., Chena, P., Hou, Z., Hydro-gen-free synthesis of 1,2-propanediol from glycerol over Cu–Mg–Al catalysts, RSC Adv. 3 (2013) 16569.doi: http://dx.doi.org/10.1039/c3ra42543f

3. Goncalves,V.,Pinto,B.,Silva,J.,Mota,C., Acetylation of glycerol catalyzed by different solid acids, Catal. Today 133-135 (2008) 673.doi: http://dx.doi.org/10.1016/j.cattod.2007.12.037

4. Tan, H., Aziz, A., Aroua, M., Glycerol production and its applications as a raw material: A review, Renew. Sust. Energ. Rev. 27 (2013) 118.doi: http://dx.doi.org/10.1016/j.rser.2013.06.035

5. Johnsona,D.,Taconi,K., The Glycerin glut: options for the value-added conversion of crude glycerol resulting from biodiesel production, Environ. Progress. 26 (2007) 338.doi: http://dx.doi.org/10.1002/ep.10225

6. Klepacova, K., Mravec, D., Kaszonyi, A., Bajus M., Etheri-fication of glycerol and ethylene glycol by isobutylene, Appl. Catal. A: Gen. 328 (2007) 1.doi: http://dx.doi.org/10.1016/j.apcata.2007.03.031

7. Melero,J.,Grieken,R.,Morale,G.,Paniagua,M., Acidic mesoporous silica for the acetylation of glycerol: synthesis of bioadditives to petrol fuel, Energy and Fuels 21 (2007) 1782.doi: http://dx.doi.org/10.1021/ef060647q

8. Ferreira,P.,Fonseca,I.,Ramos,A.,Vital,J.,Castanheiro,J., Glycerol acetylation over dodecatungstophosphoric acid immobilized into a silica matrix as catalyst, Appl. Catal. B: Environ. 91 (2009) 416.doi: http://dx.doi.org/10.1016/j.apcatb.2009.06.009

9. Dosuna-Rodriguez,I.,Adriany, C., Gaigneaux, E., Glycerol acetylation on sulphated zirconia in mild conditions, Catal. Today 167 (2011) 56.doi: http://dx.doi.org/10.1016/j.cattod.2010.11.057

10. Dosuna-Rodriguez, I.,Gaigneaux,E., Glycerol acetylation catalysed by ion exchange resins, Catal. Today 195 (2012) 14.doi: http://dx.doi.org/10.1016/j.cattod.2012.04.031

11. Jagadeeswaraiah,K.,Balaraju,M.,SaiPrasad,P.,Linga-iah, N., Selective esterification of glycerol to bioadditives over heteropoly tungstate supported on Cs-containing zir-conia catalysts, Appl. Catal. A: Gen. 386 (2010) 166.doi: http://dx.doi.org/10.1016/j.apcata.2010.07.046

12. Ferreira,P.,Fonseca,I.,Ramos,A.,Vital,J.,Castanheiro,J., Acetylation of glycerol over heteropolyacids supported on activated carbon, Catal. Commun. 12 (2011) 573.doi: http://dx.doi.org/10.1016/j.catcom.2010.11.022

13. Liao, X., Zhu, Y., Wang, S., Li, Y., Producing triacetylglyc-erol with glycerol by two steps: Esterification and acetyla-tion, Fuel Proc. Technol. 90 (2009) 988.doi: http://dx.doi.org/10.1016/j.fuproc.2009.03.015

14. Guerrero-Perez,M., Rosas, J., Bedia, J., Rodriguez-Mira-sol,J.,Cordero,T., Recent Inventions in Glycerol Transfor-mations and Processing, Recent Patents on Chem. Engg. 2 (2009) 11.doi: http://dx.doi.org/10.2174/2211334710902010011

15. Granados,M.,Alba-Rubio,A., Vila, F.,Alonso,D.,Mari-scal, R., Surface chemical promotion of Ca oxide catalysts in biodiesel production reaction by the addition of mono-glycerides, diglycerides and glycerol, J. Catal. 276 (2010) 229.doi: http://dx.doi.org/10.1016/j.jcat.2010.09.016

16. Russbueldt, B., Hoelderich, W., New rare earth oxide cata-lysts for the transesterification of triglycerides with metha-nol resulting in biodiesel and pure glycerol, J. Catal. 271 (2010) 290.doi: http://dx.doi.org/10.1016/j.jcat.2010.02.005

17. Zhou, C. H., Beltramini, J., Fan, Y. X., Lu, G. Q., Chemo-selective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals, Chem. Soc. Rev. 37 (2008) 527.doi: http://dx.doi.org/10.1039/B707343G

18. Gallezot,P., Catalytic routes from renewables to fine chem-icals, Catal. Today 121 (2007) 76.doi: http://dx.doi.org/10.1016/j.cattod.2006.11.019

19. Ferreira,P.,Fonseca,I.,Ramos,A.,Vital,J.,Castanheiro,J., Esterification of glycerol with acetic acid over dodeca-molybdophosphoric acid encaged in USY zeolite, Catal. Commun. 10 (2009) 481.doi: http://dx.doi.org/10.1016/j.catcom.2008.10.015

S. A. RANE et al., Esterification of Glycerol with Acetic Acid over Highly Active and…, Chem. Biochem. Eng. Q., 30 (1) 33–45 (2016) 45

20. Goncalves, C., Laier, L., Jose da Silva, M., Novel Esterifi-cation of Glycerol Catalysed by Tin Chloride (II): A Recy-clable and Less Corrosive Process for Production of Bio-Additives, Catal. Lett. 141 (2011) 1111.doi: http://dx.doi.org/10.1007/s10562-011-0570-x

21. Dosuna-Rodriguez,D.,Adriany,C.,Gaigneaux,E., Glycer-ol acetylation on sulphated zirconia in mild conditions, Catal. Today 167 (2011) 56–63doi: http://dx.doi.org/10.1016/j.cattod.2010.11.057

22. Pudi,S.,Mondal,T.,Biswas,P.,Biswas,S.,Sinha,S.,Con-version of Glycerol into value-added products over Cu–Ni catalyst supported on γ-Al2O3 and activated carbon, Int. J. Chem. React. Eng. 12 (2014) 151.doi: http://dx.doi.org/10.1515/ijcre-2013-0102

23. Khayoon, M., Hameed, B., Acetylation of glycerol to biofu-el additives over sulfated activated carbon catalyst, Biore-source Technol. 102 (2011) 9229–9235.doi: http://dx.doi.org/10.1016/j.biortech.2011.07.035

24. Ji, X., Chen, Y., Wang, X., Liu, W., Sulphate-functionalized multi-walled carbon nanotubes as catalysts for the esterifi-cation of glycerol with acetic acid, Kinet. Catal. 52(4) (2011) 555.doi: http://dx.doi.org/10.1134/S0023158411030104

25. Sanchez, J., Hernandez, D., Moreno, J., Mondragon, F., Fernandez, J., Alternative carbon based acid catalyst for selective esterification of glycerol to acetylglycerols, Appl. Catal. A: Gen. 405 (2011) 55.doi: http://dx.doi.org/10.1016/j.apcata.2011.07.027

26. Zhou,L.,Nguyen,T.,Adesina,A., The acetylation of glyc-erol over amberlyst-15: Kinetic and product distribution, Fuel Proc. Technol. 104 (2012) 310.doi: http://dx.doi.org/10.1016/j.fuproc.2012.06.001

27. Zhou,L.,Al-Zaini,E.,Adesina,A., Catalytic characteristics and parameters optimization of the glycerol acetylation over solid acid catalysts, Fuel 103 (2013) 617.doi: http://dx.doi.org/10.1016/j.fuel.2012.05.042

28. Valente, J., Cantu, M., Cortez, J., Montiel, R., Bokhimi, X., Lopez-Salinas,E., Preparation and characterization of sol-gel MgAl hydrotalcites with nanocapsular morphology, J. Phy. Chem. C 111 (2007) 642.doi: http://dx.doi.org/10.1021/jp065283h

29. Yang, T., Chang, T., Yeh, C., Acidities of sulfate species formed on a superacid of sulfated alumina, J. Mol. Catal. A: Chem. 115 (1997) 339.doi: http://dx.doi.org/10.1016/S1381-1169(96)00283-X

30. Dumitriu, E., Hulea, V., Effects of channel structures and acid properties of large-pore zeolites in the liquid-phase tert-butylation of phenol, J. Catal. 218 (2003) 249.doi: http://dx.doi.org/10.1016/S0021-9517(03)00159-3

31. Khandan, N., Kazemeini, M., Aghaziarati, M., Determin-ing an optimum catalyst for liquid-phase dehydration of methanol to dimethyl ether, Appl. Catal. A: Gen. 349 (2008) 6.doi: http://dx.doi.org/10.1016/j.apcata.2008.07.029

32. Karim, M., Rahman, M., Miah, M., Ahmad, H., Yanagisa-wa,M., Ito,M.,Synthesis of γ-alumina particles and sur-face ccharacterization, The Open Colloid Sci. J. 4 (2011) 32.doi: http://dx.doi.org/10.2174/1876530001104010032

33. Dean, J., (1999) Lange’s handbook of chemistry 15th ed., McGraw-Hill, New York

34. Mekhemer, G. A. H., Khalaf, H. A., Mansour, S. A. A., Nohman, A. K. H., Sulfated Alumina Catalysts: Conse-quences of Sulfate Content and Source, Monatshefte fur Chemie 136 (2005) 2007.doi: http://dx.doi.org/10.1007/s00706-005-0374-z

35. Gandarias, I., Requies, J., Arias, P., Armbruster, U., Martin, A., Liquid-phase glycerol hydrogenolysis by for-mic acid over Ni–Cu/Al2O3 catalysts, J. Catal. 290 (2012) 79.doi: http://dx.doi.org/10.1016/j.jcat.2012.03.004

36. Kang,M.,Song,M.,KimT.,Kim,K., γ-alumina supported Cu-Ni bimetallic catalysts: Characterization and of 1,3-Bu-tadiene, Can. J. Chem. Eng. 80 (2002) 63.doi: http://dx.doi.org/10.1002/cjce.5450800107

37. Goncalves, C., Laier, L., Cardoso, A., Silva, M., Bioadditive synthesis from H3PW12O40-catalyzed glycerol esterification with HOAc under mild reaction conditions, Fuel Proc. Technol. 102 (2012) 46.doi: http://dx.doi.org/10.1016/j.fuproc.2012.04.027